Conventional powder metallurgy has been used for many years to produce a variety of tungsten-based alloys with densities approaching that of pure tungsten (19.3 g/cc). These alloys are collectively referred to as “WHA's” (i.e., tungsten heavy alloys) and typically have densities in the range of approximately 15 g/cc to approximately 18 g/cc). Examples of these alloys include, but are not limited to, W—Cu—Ni, W—Co—Cr, W—Ni—Fe, W—Ni, and W—Fe. Regardless of which alloy family is to be produced, the basic procedure is the same: appropriate proportions, chemical compositions and particle sizes of metallic powders are blended together, pressed into desired shapes, and finally sintered to yield consolidated material with desired physical and mechanical properties. WHA alloys are widely produced for use in such articles as counterweights, radiation shields, aircraft stabilizers, and ballast weights.
Following the initial processing described above, it is common practice to convert the sintered shapes to products of final dimensions and finishes by such processes as forging, swaging, drawing, cropping, sawing, shearing, and machining. Operations such as these inherently produce a variety of metallic scrap, such as machine turnings, chips, rod ends, broken pieces, rejected articles, etc., all of which are generated from materials of generally high unit value because of their tungsten content. Despite this value, however, it has proven difficult to recycle this WHA scrap other than by methods that employ chemical processes to recover the tungsten, which then must be reformed into a WHA. Often times, these processes also produce chemical waste streams, which raise environmental and health concerns as well as requiring treatment and disposal.
Examples of these chemical recovery processes include oxidation/reduction, anodic dissolution of secondary elements and dissociation by molten zinc. Oxidation/reduction involves oxidizing the WHA scrap in a high-temperature oxidizing environment that converts the alloy into mixed metal oxides, in which tungsten is present as tungsten trioxide. The mixed metal oxides are separated via chemical processes to isolate the tungsten trioxide alone or in combination with selected ones of the metal oxides. The isolated oxides are subsequently reduced to elemental tungsten or a mixture of metallic powders. This process requires special furnaces operating at temperatures in excess of 1000° C. in a dry hydrogen atmosphere free of any oxygen-containing substances. The reduction reaction consists of the reaction of hydrogen with the metal oxides, thereby producing water and elemental metal as products. Although this process is widely used, it is energy-intensive, relatively dangerous because of the high-temperature hydrogen used therein and expensive. Also, when larger WHA scrap pieces are used, the process becomes impractical because of the low surface-to-volume geometries of such pieces of WHA. Essentially, it is necessary to oxidize the pieces for a time, mechanically remove the oxide from the surfaces, and then repeat the process until the metal has been fully oxidized to its core.
Another chemical method is anodic dissolution, which consists of placing solid pieces of WHA scrap in a perforated stainless steel basket. The basket forms the anode in an electrolytic cell, with the electrolyte being sulfuric acid. Electrolysis at controlled voltages produces dissolution of the secondary elements in the WHA scrap, such as iron, nickel, copper, etc., and leaves behind a porous friable skeletal structure of tungsten-rich material that may be ground to powder for subsequent recycling. In addition to being relatively slow and energy-intensive, it also generates sulfuric acid wastes contaminated with undesirable metallic ions.
One other known chemical process is referred to as dissolution of secondary elements by molten zinc and involves exposing WHA scrap to molten zinc for periods of time sufficient to cause dissolution of elements other than tungsten in the liquid metal phase. The pregnant zinc liquid is physically separated from the solid tungsten residues, then vaporized and distilled to reclaim the various secondary metals and the zinc itself, which is subsequently recycled. This method has the disadvantages of potential pollution and health problems associated with handling zinc vapors and chemical waste disposal concerns associated with the secondary metals, several of which are viewed as “toxic heavy metals.”
Therefore there is a need for an economical method for recycling WHA materials, and especially WHA scrap, into useful articles. The present invention relates to methods for producing medium-density articles from recovered high-density tungsten alloy (WHA) material, and especially from recovered WHA scrap. In one embodiment of the invention, the method includes forming a medium-density alloy from WHA material and one or more medium- to low-density metals or metal alloys. In another embodiment, medium-density grinding media, such as formed from the above method, is used to mill WHA scrap and one or more matrix metals into particulate that may be pressed and, in some embodiments, sintered to form medium-density articles therefrom.
Many other features of the present invention will become manifest to those versed in the art upon making reference to the detailed description which follows and the accompanying sheets of drawings in which preferred embodiments incorporating the principles of this invention are disclosed as illustrative examples only.